Generally described, a combined cycle power plant uses a combination of a gas turbine and a steam turbine to produce electrical power. Specifically, a gas turbine cycle may be operatively combined with a steam turbine cycle by way of a heat recovery steam generator (“HRSG”) and the like.
Because the steam turbine cycle is driven from the exhaust of the gas turbine, the HRSG may not become fully operational until the gas turbine cycle has increased the steam turbine cycle to a suitable temperature. For example, the temperature of the exhaust gas gradually increases at startup as the firing temperature of the gas turbine increases. Although the hot exhaust gases from the gas turbine flow through the HRSG, a considerable period of time may elapse before an initially cold HRSG is capable of generating steam at a sufficient pressure and temperature. In conventional systems, the gas turbine thus may be kept at relatively low loads until the temperature of the HRSG increases to a level where the HRSG can generate steam at a desired pressure and temperature.
The superheated steam generated in the HRSG may be located several hundred feet away from the steam turbine or more. The piping connecting the HRSG and the steam turbine thus may be several hundred degrees cooler than the superheated steam being generated. Flowing superheated steam through pipes with cooler metal temperatures with respect to the steam, however, may cause a temperature reduction of the steam upon admission into the steam turbine. This drop in temperature may lead to a loss of equipment life or delays in allowing steam admission to the turbine due to inadequate time for the pipes to warm up. Similarly, if steam admission is delayed to allow the pipes to warm up gradually, increased operational costs may be incurred as the generated steam is bypassed to the condenser or otherwise as opposed to being used to generate work. There is thus a desire for an improved heat recovery steam generator system and associated piping startup and warming procedures. Preferably, such improved systems and procedures should mitigate the stress within the piping while also providing improved start up times.